1. Field of the Invention
The present invention relates to a method for controlling charge to a secondary battery such as a nickel-metal hydride secondary battery mounted on an automated guided vehicle.
2. Description of the Prior Art
Hitherto, an automated guided vehicle used for carrying goods in a factory, a warehouse, or the like, for the purpose of automation or laborsaving, includes a battery to be mounted as a power source of an electric motor for running and a drive source for load, and is controlled so as to run automatically on various running paths along a guide line such as a guide tape adhered to a floor of a factory, a warehouse, or the like.
When the battery is required to be charged or when no request to carry goods is made by an operation control station, the automated guided vehicle moves toward a charge station provided at the inner side etc. of the running path as a destination and is connected to a charger and the battery is charged at the charge station.
As the battery mounted on such an automated guided vehicle, conventionally a lead-acid battery was used. However, the lead-acid battery needs a long charging time, facilities for charging or replacing the battery, and staff to maintain the battery. For this reason, recently, instead of the lead-acid battery, an alkaline battery such as a nickel-metal hydride secondary battery, which is capable of rapid charge at the charge station and does not require the maintenance, has been used.
In such an alkaline battery for an automated guided vehicle, since the rapid charge for about a few minutes is carried out for the purpose of minimizing the stopping time at the charge station and improving the carrying efficiency, the charging rate is set to 0.5 C to 4.0 C, which is much larger than with that of an alkaline battery for an electric vehicle whose charging rate is low and charging time is long. Therefore, if the alkaline battery for an automated guided vehicle is charged until the remaining capacity (state-of-charge: SOC) reaches 100%, the internal pressure of the battery is increased, which may lead to such a problem that a relief valve is operated and gas leakage occurs.
Furthermore, in the alkaline battery for automated guided vehicles, since the range of the actually used SOC is a narrow range of a middle region, a memory effect by which the terminal voltage of the battery is lowered, generally occurring at the discharge side, occurs also at the charge side, and thus the terminal voltage is increased in the used middle region of SOC.
If such a phenomenon occurs, charge acceptability with respect to an alkaline battery is deteriorated, or the terminal voltage is increased, thereby resulting in a false detection that the terminal voltage reached a predetermined upper limit charging voltage.
With the foregoing in mind, it is an object of the present invention to provide a method for controlling charge to a secondary battery for an automated guided vehicle that prevents an operation of a valve during charging and suppresses a false detection of the upper limit charging voltage due to the memory effect at the charge side.
In order to achieve the above-mentioned object, a method for controlling charge to a secondary battery for an automated guided vehicle of the present invention is a method for controlling a charging amount of the secondary battery mounted on the automated guided vehicle and used as a power source of an electric motor for running the vehicle. The method includes charging the secondary battery at a charging current value of not less than 0.5 C and not more than 4.0 C; detecting current that flows in the secondary battery and calculating the remaining capacity by accumulating at least the detected current; and completing the charge to the secondary battery when the calculated remaining capacity is not less than the threshold value, which is pre-set to not less than 60%, and not more than 95%.
In this method for controlling charge to a secondary battery for an automated guided vehicle, it is preferable that the method includes detecting a temperature of the secondary battery; detecting an output voltage of the secondary battery; calculating a voltage gradient of the detected output voltage with respect to the time; and completing the charge to the secondary battery when, excluding the time right after the charge is started, the calculated voltage gradient is not less than a first threshold value corresponding to the detected temperature and the detected output voltage is not less than a second threshold value corresponding to the detected temperature.
It is preferable that the first threshold value and second threshold value are represented by a first functional equation and a second functional equation having variables of the detected temperature, respectively, and the first and second functional equations are determined in accordance with the intended set value of the remaining capacity at the time the charge is completed.
Furthermore, it is preferable that the first and second functional equations are determined respectively so that the first and second threshold values are reduced as the detected temperature is higher.
Furthermore, it is preferable that the first and second functional equations are determined in accordance with the charging current value.
In this case, it is preferable that the first and second functional equations are determined so that the first and second threshold values are increased as the charging current value is larger.
Furthermore, it is preferable that the first and second functional equations are expressed respectively by a linear equation.
Furthermore, it is preferable that the charge is considered complete upon satisfying at least one case selected from the group consisting of a case where the calculated remaining capacity is not less than the threshold value that is pre-set to be not less than 60% and the charging time reaches a predetermined period of time; a case where the detected temperature reaches a predetermined upper limit temperature; and a case where a temperature gradient of the detected temperature with respect to the time is calculated and the calculated temperature gradient reaches a predetermined value.
According to the above-mentioned method, it is possible to suppress the occurrence of the memory effect at the charge side and to resolve the false detection of the upper limit charging voltage, thus enabling an extremely stable charge control. Furthermore, by setting the remaining capacity (SOC) for completing the charge to be not more than 95%, the operation of the relief valve due to the increase of the internal pressure of the battery at the late-stage of charging can be suppressed.